This invention relates to a multi-beam type electron gun assembly having a plurality of electron guns and more particularly to an electron gun assembly for use in a multi-beam type cathode ray tube including an improved grid structure wherein the identical grid electrodes of respective electron guns are formed as an integral structure.
Generally, a three-beam type electron gun assembly as a typical example of the multi-beam type electron gun assembly for use in the cathode ray tube is often utilized in a colour picture tube in which three electron guns are disposed in the neck of the envelope of a colour picture tube which comprises a panel, a funnel and the neck. Each electron gun comprises a first grid electode provided with a cathode or a cathode heater and a plurality of other grid electrodes aligned in the direction from the neck to panel of the envelope of the picture tube to follow the first grid electrode. These grid electrodes are impressed with the predetermined potential so as to form a prefocussing lens system and a main focussing lens system between the predetermined ones of the electrodes.
In the prior art construction of the electron gun assembly of the type referred to above it is usual to assemble three independent electron guns together on a supporting member such as made of beads glass by taking a triangle or in-line position. However, in assembling the three independent electron guns together on the beads glass supporting member there are such difficulties that it is necessary to align respective electron guns in the axial direction and to assemble the electron guns in correct relative positions. Moreover, this construction increases the number of component elements and hence the cost of assembling them so that the size and the weight of the electron gun assembly cannot be reduced beyond certain limits.
To obviate these difficulties, in recent years, it has been proposed to construct as an integral unit a particular group of grid electrodes among the grid electrodes of respective electron guns, the grid electrodes to be grouped being impressed with the same potential and presenting the same function. According to this construction, it is possible to decrease the number of the component elements, to simplify the assembling work and to reduce the size and weight relative to the prior art construction. Especially integral formation of respective identical grids constituting a main focussing lens results in an elevated accuracy for assembling an electron gun assembly, thus the characteristics of the electron gun assembly are enhanced to a great extent. This integral grid is formed by the following method. First a cup shaped member is formed, the bottom of which is provided with three openings arranged in the predetermined position, each opening being used for the passage of electron beams respectively traveling within each electron gun assembled, and has a predetermined diameter. Then another cup shaped member is formed having a bottom provided with openings coaxial with the openings of the first mentioned cup shaped member, each opening being used for the same purpose as in said first mentioned cup shaped member. These two cup shaped member are disposed to oppose each other with a predetermined spacing between the bottoms and the openings thereof correspond with each other thus assembling the integrally formed grid electrode. According to this construction, the openings of respective bottoms will not be positioned at an equal distance from the side walls of the opposing cup shaped members unless the cross section of the cup shaped member is circular and the axes of openings coincides with that of said cup shaped member. Under these conditions, when a predetermined potential is impressed upon the pair of cup shaped members spaced from each other a predetermined distance, the potential distribution between opposing openings will necessarily be affected by the side walls of the opposing cup shaped members. In other words, the potential distribution, which otherwise would be rotationally symmetrical with respect to the axes of the openings, will be greatly affected by the edge effect of the side walls and that of the openings. Accordingly to obviate such an adverse effect it was proposed to provide a cylindrical tube along the peripheries of the openings axially towards the inside of the cup shaped members so as to eliminate the above-mentioned effect. To form the cup shaped member, press work is desirable in view of easiness and accuracy in the process. However, it is impossible to obtain such cup shaped member that has a useful edge integrally formed along the peripheries of the openings having the same function as the abovementioned cylindrical tube because the depth attained through the press process has to be limited with the relationship between the diameter of the openings and the permissible pressed depth in connection with a raw material and so forth. Accordingly the effect of such press work is only to increase the breakdown voltage of the opposing grid structures. That is to say, constituting the electron lens system with the grid in which inadequate pressed depth is achieved compared with the opening diameter results in such nonuniform potential distribution between opposed openings as to cause an asymmetry with respect to the axis of the electron lens which in turn results in the increase in the astigmatism of the lens.